Page 269 - Carbonate Facies in Geologic History

256 Permo-Triassic Buildups and Late Triassic Ecologic Reefs

Table VIII-2. Comparison of biotas of Permo-Triassic buildups
Permian Reef Complex Dolomites and Wetterstein Norian-Rhaetic
Limestone

Syconid sponges are major Syconid (Sphinctozoan) are Only Inozoan sponges
large organisms major large organisms; more Peronidella
than 50% of genera are the
same as in the Permian. Clear
phylogenetic relations are seen
with Permian
"Encrusting Hydrozoan" of Tubiphytes is most important 1Ubiphytes is rare but
Newell et al., is very binding organism reappears in Late Jurassic
important sediment- reefs
binding organism
(Tubiphytes)
Lamellar stromatolitic Only wavy lamellar crusts Lamellar stromatolitic
crusts lining cavities and lining cavities. Coarse druse in crusts
knobby stromatolites. cavities
Coarse druse in cavities
No colonial corals Colonial corals not common Colonial corals are the most
but range in size from small to important large organisms.
about two meters. Forms are of Mainly organpipe forms,
dendroid construction, "Thecosmilia", up to 10 m
"Thecosmilia" types in micritic high
bedded limestone
Hydrozoans Hydrozoans-spongiomorphs Hydrozoan spongiomorphs
(stromatoporoids) present (stromatoporoids) present may be dominant with
corals. Large masses
Unusually large and bead- Teutioporella, Poikiloporella Dasyc1adaceans also
like dasycladacean algae: and Diploporella. Common in important as sands in
Mizzia and Macroporella, lime sands at bank margin and immediate backreef
forming grainstones at top in bank interior
of buildup

It is notable that the Type I bank margins in both the Permian and in the
Dolomites were commonly eroded and afforded a large supply of exotic blocks
down the slopes. If most of the organisms and their matrix of fine sediment
indicate growth in quiet water at or below wave base, what is the origin of the
blocks? Perhaps periodic drops in sea level exposed and partly cemented the tops
of the banks so that hard chunks could be eroded off by wave action during
periods of shallow water. Climatic similarity probably has much to do with the
generally similar sediments in the Permian and Triassic. The redbeds, dune sands,
dolomites, evaporites, and important thick carbonate banks, large sponges, dasy-
cladacean algae and corals, and interior tidal flat-lagoon sediments with spectacu-
lar vadose diagenetic features all indicate strongly seasonal climate with extended
warm and arid seasons-probably interspersed with wet periods. This resulted in
unusually rapid cementation of the shelf margin and downslope sediments. The
striking diagenetic effects imposed on the cyclic sediments within the lagoons of
the banks are discussed in Chapter X. The tidal flat and restricted marine strata of
the Permian and Triassic lagoons resemble each other just as much as do those of
the shelf margins.